The invention relates to a method of decorating a blank which is to be shaped typically into a hollow packinging element or receptable, and also to the use of this method for the manufacture of bodies of preserves cans and other shaped packaging elements or receptacles, and also to the products obtained thereby.
It is known to decorate a blank which is intended to be shaped into a metal receptance, for example by drawing and ironing in order to obtain a desired decoration on the outside of the shaped receptacle, the prior decoration of the blank taking into account the shaping which this blank is to undergo.
For instance, the document FR-B-2535858 (corresponding to U.S. Pat. No. 4,556,312) describes a device for printing circular blanks intenede to be drawn into cans, according to a predeformed image by an optical method, the optical system comprising a face cut on the basis of the evolutions of distances from points on the lateral surface of the can at the contour of the bottom of the can between the blank and the drawn product.
On the other hand, the document EP-A-0215718 describes a method of and an apparatus for predeforming the printed image decorating a mechanically drawn metal can . For predeformations corresponding to variable elongations due to the drawing of the walls of the can, this articulated bar apparatus uses a template, the bearing edge of which is defined by means of the articulated bar and the distances between successive reference lines on a sample cut into the side wall of a test can.
In turn, the document EP-A-0202928 describes a method in which the predetermined image obtained on the blank is based on computerised conversion of the original rectangular image into an annular image having the same surface area, which is used for printing on the blank. This conversion is based on a calculation of the polar co-ordinates of each point on the annular image of the blak from rectangular co-ordinates of a corresponding point on the original image. A correction according to the anisotropy of the elongation of the blank is alluded to, with no more specific details.
In circular metal preserves cans obtained by deep drawing and which typically have at the bottom a height: diameter ratio greater than 0.5, there are local differences in the elongation of the lateral wall, corresponding to the "drawing ears" phenomenon, and the harder the blank, the more marked this phenomenon will be. These irregularities appear towards the top of the cans and are reflected in deformations of the decoration, lines (marks, characters, figures) or designs situated at this level. In the case of cans which have a non-circular contour, for example so-called rectangular cans, there are also considerable localised irregularities arising from contraction of the wall parallel to the bottom and which can be superimposed on the aforementioned irregularities in elongation.
The three prior art methods mentioned hereinabove apply one and the same predeformation to all points of the drawn product which are situated at the same level. They do not make it possible to correct the previous irregularities, which in practice leads either to avoiding excessively anisotropic materials or to tolerating distortions or inaccuracies in the decor of the drawn product in relation to the desired decor, or yet in adopting flat tin areas in the zones which are sensitive to the aforesaid irregularities.
The Applicants have sought to perfect a method of decorating the blank which makes it possible to overcome such irregularities and the constraints and imperfections associated with them.